Presently and in the years ahead, the use of coal for energy is expected to become more and more economical and popular. Such use is critical if we are to relieve our dependency on imported petroleum and other liquid/gas fuels that continue to increase in price. In the past, coal has been shipped by railway, barge and trucks. Such methods of shipping suffice for relatively small quantities of coal. However, as the use of coal increases, it becomes more and more important to provide more economic methods to transport this fuel.
For many years, petroleum and petroleum products have been transported through pipelines over hundreds and hundreds of miles. The pipelines are capable of transporting massive volumes of fuel to the urban centers of the country at a fraction of the cost of other transportation modes. It, therefore, is not surprising that coal slurry pipelines for transporting coal suspended in water or other carrier liquids have been proposed. In fact, many successful slurry pipelines are in operation today. They, however, are mostly for conveying coal over a relatively short distance to a power plant from an adjacent mine.
A major drawback of a slurry pipeline is the abrasive effect of the solid coal and rock particles on the pump. Particles tend to intrude and lodge between the pumping cylinder and the sealing lip of the piston as well as between the pump shaft and the main seal in the cylinder head. The lodged particles can quickly damage the flexible seals and may, under certain conditions, quickly wear the cylinder and/or shaft to the point where an effective sealing relationship is lost. This results in the loss of pumping pressure. The pump must then be repaired, such as by inserting a new cylinder liner and/or piston, and of course, the seals must also be replaced. This repair and replacement greatly increases the cost of operation of slurry pumps. Thus, solving this problem would greatly enhance the economic feasibility of long-distance slurry pipelines.
Proposals in the past include injecting a liquid into the pump to form a more efficient sealant for the piston. If the metered sealant liquid, such as water, is fed at a sufficient pressure, the liquid seeps from around the seals to dislodge the solid particles before damage to the seals, shaft or cylinder can occur.
An improved self-flushing piston assembly designed to supply metered liquid supply to the periphery of the piston is shown in my issued U.S. Pat. No. 4,476,771, issued Oct. 16, 1984, and entitled Self-Flushing Piston Assembly for Slurry Pump. The piston assembly includes a slurry pump head having a pumping and lost motion piston coupled together. Flushing liquid is drawn into a variable volume chamber between the pistons during the suction or return stroke. On the power stroke, flushing liquid is ejected around the sealing periphery of the piston on the slurry side to prevent particle intrusion of the seal. An improvement invention wherein the output of the pump is substantially doubled is shown and claimed in my copending application, Double Acting Self-Flushing Pump, Ser. No. 726,181, filed Apr. 24, 1985.
The new pump assemblies of these issued patents, as well as others previously known in the art, however, fail to provide particle intrusion protection to a seal around the pump shaft. A need is, therefore, identified for an improved self-flushing fluid seal assembly for preventing particle intrusion into the main seal around a slurry pump shaft so as to reduce wear and, thereby, further reduce slurry pump maintenance and operating costs.